In producing molten iron by a blast furnace, iron ore as iron raw material, coke as fuel, lime stone as by-product, etc. are introduced into the furnace from the top thereof, and hot blast is blown into the furnace from a tuyere in the lower portion thereof to burn the coke, so that the iron ore is reduced by the generated reducing gases mainly comprising CO and heat energy. As a result, the iron content of the iron ore becomes the main component of molten iron, while the gangue minerals of the iron ore, and the ashes of the coke become slag together with the limestone, both of which are periodically discharged from a tap hole and a slag hole, respectively, of the lower portion of the furnace. In the blast furnace, the molten iron is produced by the process of high-temperature reaction of the raw material and the reducing gases, and thus it is very important to maintain a stationary state while maintaining a material balance and a heat balance. There is a strong demand for maintaining the stability of furnace conditions in the operation of the blast furnace.
Therefore, in order to maintain the stable conditions of the blast furnace, it is an essential condition to sufficiently control heat conditions in the furnace.
The heat conditions in the blast furnace is divided into the level of heat conditions and the transition of heat conditions, and is one of items regarded as the most important information which reflects the in- furnace conditions such as reaction in the furnace, etc. The level of heat conditions and the transition of the heat conditions basically significantly affect the temperature of molten iron. Therefore, in order to stabilize the operation of the blast furnace and in order to decrease the unit fuel consumption, and in order to improve productivity and quality of molten iron, it is very important to measure, correctly and precisely, the temperature of the molten iron with a short time delay, to control the heat conditions in the furnace, on the basis of the information of the temperature measurement, and adjust, correctly and precisely, the temperature of the molten iron, to a target level. However, for the heat conditions in the blast furnace, conventionally, the level of the heat conditions in the furnace is evaluated by the temperature of the molten iron after tapping, and the heat conditions transition is evaluated and inferred from information from the various sensors arranged in the blast furnace.
(1) The Level of Heat Conditions.
Generally, in tapping in a blast furnace, molten iron is injected from the tap hole, passed through a long runner having a length of as long as about 20 m, and flows into a skimmer. As the position and method of measuring the temperature of the molten iron, a method is conventionally employed in which the molten iron and slag are separated by the skimmer on the basis of the difference in a specific gravity so that the slag floats on the molten iron, and then the temperature of the molten iron is measured. The temperature is measured by using an immersion type thermocouple thermometer. On the inner surface of the runner is formed a runner comprising a monolithic refractory. Therefore, the temperature of the tapped molten iron is decreased due to heat extraction by heat conduction to the runner and heat radiation to air in the course of passage through the runner. In tapping, the diameter of the tap hole is increased due to wearing by the slag, and thus the tapping rate changes with the passage of the tapping time.
The temperature of the molten iron measured in the skimmer is affected by heat extraction from the molten iron in the runner and a change in the tapping rate (t/min) to greatly change during the time from the start to end of tapping. The temperature is generally low in the initial stage of tapping, and then gradually increases to the highest temperature in the last stage of tapping. Conventionally, the highest temperature is used as the temperature of the molten iron.
The blast furnace is generally operated so that the tapping rate is slightly higher than the production rate of the molten iron in the furnace. Therefore, tapping from the predetermined tap hole is performed for about 3 to 4 hours at a time, and then the tap hole is closed to wait until a molten iron is produced and again accumulated in the vicinity of the tap hole. However, during this waiting time, another tap hole is opened for tapping in the same way. Generally, tapping is carried out through an opposite tap hole of the furnace for about 3 to 4 hours at a time. During tapping through the other tap hole, the temperature of the runner provided on the predetermined tap hole is decreased. Therefore, when measuring the temperature of the molten iron in the skimmer during next tapping, the temperature changes in such a manner that it is low in the initial stage, and reaches the highest temperature in the last stage. However, such changes in the temperature of the molten iron measured in tapping are not constant, and greatly vary from one tapping to another, as shown later in FIG. 11.
FIG. 11 shows an example of measurement results of the temperature of a molten iron by a conventional method. FIG. 8 shows the results of measurement only in tapping in which the highest temperature was actually 1500 to 1510.degree. C. in 8 to 12 temperature measurements of the molten iron at one tapping. This graph indicates that in the conventional method of measuring the temperature of the molten iron, even in cases at the same level of highest temperature, variations occur in the first temperature measurements, and the rising patterns from the initial stage to the last stage at the highest temperature are not constant. Therefore, it is difficult to not only infer the highest temperature from the first temperature measurement, but also infer the highest temperature from the temperature measurement in the course from second measurement to later measurement.
Also, since the runner of the spout comprising a monolithic refractory is worn by a slag flow, the runner is generally changed for every 2 to 3 weeks. In first tapping after the runner is 7repaired, the sensible heat of the tap runner is small, and thus heat extraction from the molten iron to the tap runner is further increased. In measuring temperature of the molten iron in first tapping after the tap runner is constructed, the initial measurement is further decreased.
In accordance with conventional temperature measurement of the molten iron in the skimmer, the temperature of the molten iron changes in such a manner that it increases with the passage of time from the start of tapping, and reaches the highest temperature in the last stage. However, for the above-mentioned reasons, the rising curve of temperature greatly varies from one tapping to another.
In the temperature of the molten iron in the skimmer in the initial stage of tapping, the temperature of the molten iron in the furnace greatly decreases with low precision, and a long time is required until the temperature of the molten iron in the skimmer reaches the tapping temperature and is stabilized. Furthermore, even if the measurement in the skimmer is corrected by data from many operations, it is difficult to correctly infer the temperature of the molten iron in the furnace. It is also difficult to know the temperature with a short time delay.
For the above reasons, in the conventional method of measuring the temperature of a molten iron, for the temperature of the molten iron permitting evaluation of t he level of the heat conditions, only 1 item of data can be obtained in 3 to 4 hours required for one time of measurement. Therefore, the conventional method has problems as means for evaluating the heat conditions in the furnace.
(2) Heat Conditions Transition
As described above, the transition pattern of the temperature of the molten iron varies from one tapping to another, and large variations occur in the patterns. Furthermore, since information of the temperature of the molten iron at a measurement time in the skimmer is accompanied with a time delay for inferring heat conditions transition, the action of controlling heat conditions is also delayed. Therefore, information of only the temperature of the molten iron measured by the conventional measurement method causes a delay in the action on the operation of a blast furnace, thereby making impossible the operation under stable conditions. Therefore, in order to prevent a delay in the action, information from various sensors is employed for determining the heat conditions transition. Typical examples of such sensors include an embedded tuyere sensor comprising a thermocouple embedded in the vicinity of the tuyere of the blast furnace, a furnace exhaust gas sensor and the like. These sensors are provided for measuring the temperature of the furnace in the vicinity of the tuyere, and the components of the furnace exhaust gases to detect a change in the heat conditions at rapid timing and measure the heat conditions transition without a time delay.
Although the absolute value of the temperature (referred to as "the embedded tuyere temperature" hereinafter) measured by the embedded tuyere sensor is significantly lower than the temperature of the molten iron, data of the temperature of the molten iron can be continuously obtained in an early stage by combination with information of other sensor values. Therefore, the embedded tuyere sensor is essential for determining the heat conditions transition. The embedded tuyere sensor is also effective for determining the heat conditions level in a degree depending upon the type of the sensor.
FIG. 12(a) shows an example of correspondence between the measurement results of the molten iron temperature by the conventional method and the measurement results of the embedded tuyere temperature during three successive times of tapping comprising A tapping, B tapping and C tapping. In the figure, information of heat conditions based on the temperature shown by each of points P.sub.1, P.sub.2 and P.sub.3 on the curve of the embedded tuyere temperature should be reflected corresponding to information of heat conditions based on the temperature shown by each of points P.sub.1 ', P.sub.2 ' and P.sub.3 ' on the measurement curve of the molten iron temperature. Namely, the gradient of the embedded tuyere temperature, which indicates a rise in temperature basically appears as a rise in the temperature of the molten iron after a predetermined time has passed, thereby causing the problem of a time delay in reflection to the measurements of the molten iron temperature by the conventional method. In addition, in the temperature curve obtained by the conventional method of measuring the molten iron temperature, the temperature of the molten iron cannot be generally correctly measured until the last stage of each tapping, and the measured temperature always rises at intermediate points. Therefore, by using the measurement results of the molten iron temperature by the conventional method, it is impossible to decide the heat conditions transition, i.e., decide as to whether the molten iron temperature tends to rise or fall. Consequently, as means for determining the heat conditions transition, temperature information obtained by various furnace sensors is conventionally used.
(3) Conventional Method of Controlling Heat Conditions
Control of the heat conditions requires information of the measurements of the molten iron temperature and various sensor values, a heat conditions estimation model using these information, and a heat conditions correction model for determining an optimum control item and the amount of control thereof on the basis of the heat conditions inferd by the heat conditions estimation model.
An example of the conventional method of controlling heat conditions is described with reference to an example of correspondence between the measurement results of the molten iron temperature by the conventional method, and the measurement results of the embedded tuyere temperature using the embedded tuyere thermocouple as a sensor, which is shown in FIG. 12(a) and FIG. 12(b). However, it is assumed that the present state is at the end of B tapping.
The heat conditions level and the heat conditions transition are determined by the method below, and ranked to infer heat conditions, and then an action is made on the basis of the action correcting rule.
1 The present heat conditions level is determined as follows. The correlation between measurement information (for example, the embedded tuyere temperature and the analytical values of the exhaust gases from the furnace top) from various sensors, and the maximum measurement of the temperature of the molten iron in the skimmer in the present tapping is determined from data of past actual operations by a statistical method using, for example, a membership function. On the basis of the correlation, the highest temperature of the molten iron, i.e., each heat conditions level, can be inferd from each of the sensor values. Thus, on the basis of the correlation between the sensor value and the heat conditions level, the heat conditions level corresponding to the value of each of sensors obtained at this time is inferred. For example, for the embedded tuyere sensor, the heat conditions level is inferd from the sensor value at the point P.sub.1 shown in FIG. 12(b).
Similarly, the correlation between the temperature measurement of the molten iron in the skimmer measured in the first tapping or intermediate tapping and the highest temperature of the molten iron in this tapping is previously determined from data of past actual operations by a statistical method using, for example, a membership function. By using this correlation, the heat conditions level is inferred from the molten iron temperature measured, for example, the measurement of the molten iron temperature at the point P.sub.1 ' shown in FIG. 12(a).
Each of the thus-inferd heat conditions levels is weighted by the predetermined method to obtain a value considered as the present heat conditions level. Some ranks are previously provided to be centered at the temperature region of the target value of the heat conditions level so that the rank of the present heat conditions level can be determined.
2 The present heat conditions transition is determined as follows. For the embedded tuyere sensor among the various sensors, the temperature gradient from point Q.sub.1 to Q.sub.2 shown in FIG. 12(b) is determined by statistical means. Similarly, for the other sensors, the temperature gradient is determined according to this method. The temperature gradient of each of the sensors is weighted by the predetermined method to infer the heat conditions transition. For the heat conditions transition, some ranks are previously provided to be centered at the region at gradient 0 (zero) so that the rank of present gradient is determined.
3 By using the heat conditions level rank and transition rank determined above in 1 and 2, respectively, the corresponding point in a matrix (i.e., an action matrix) of predetermined heat conditions level ranks and transition ranks is determined to infer the present heat conditions.
4 For the thus-inferred present heat conditions, correction is made according to the corresponding position of the present heat conditions in the action matrix. The action is performed based on the action correction rule obtained from the predetermined heat conditions correction model. The action correction rule mainly comprises the empirical rule of experts, and cannot be determined collectively. A typical factor of the action correcting operation is the amount of the steam blown into the tuyere blast, and the action amount changes with specific operation conditions in the blast furnace, particularly the raw materials used and conditions for loading the raw materials, etc., and cannot be determined collectively.
As the corrected heat conditions estimation model, for example, Japanese Examined Patent Publication No. 7-26127 discloses a method in which in the method of inferring the heat conditions level and heat conditions transition from the measurements of the molten iron temperature and information of sensor values, fidelity is introduced to infer heat conditions by using a three-dimensional function of the heat conditions level or heat conditions transition inferd from the molten iron temperature, information of sensor values, and fidelity, which are shown on three axes.
However, the conventional method of controlling blast heat conditions has the following problems:
Since the temperature of a molten iron is measured in the skimmer, for the above reasons, only the highest temperature of the molten iron measured in the last stage of tapping can be used as a correct heat conditions level. Namely, the correct temperature of the molten iron cannot be obtained until the last stage of each tapping, and thus reliable data of the heat conditions level can be obtained only once for about 3 to 4 hours. This decreases the precision of the inference of the heat conditions level.
Furthermore, the measurements of the temperature of the molten iron in an intermediate period of tapping have low reliability, and thus cannot be used as data for correcting the heat conditions transition. In addition, data with high reliability for inferring the heat conditions level can be obtained at a low frequency, and thus has a long time delay as data for inferring the heat conditions transition. Therefore, the variations in the conventional measurements of the temperature of the molten iron cannot be used as data for correcting the heat conditions transition.
On the other hand, reliable information superior to the highest temperature information of the molten iron cannot be obtained as the heat conditions level or the heat conditions transition from various sensor values conventionally used for controlling heat conditions in operations of the blast furnace, for example, the embedded tuyere temperature, the analytical values of the gases at the furnace top, the shaft temperatures at many positions, etc., as described above with respect to the embedded tuyere temperature.